Emulsions and Emulsification
An emulsion, more precisely a liquid/liquid emulsion, is a two-phase system of two immiscible liquids (usually, incompletely immiscible), with one of the phases being dispersed in the form of droplets in the other phase. Emulsions are preferred forms for the cosmetic and personal care products because they allow for the combination of otherwise immiscible ingredients and can provide significant economic advantages. Also, the public favors the feel and look of the emulsions. For these reasons, among others, a large proportion of modern cosmetic and personal care products are emulsions, and the process of producing emulsions (or emulsification) is a very important process in the cosmetic and personal care industries.
As described, emulsions' two liquid phases do not form a single phase upon mixing. It is believed that the primary reason for this immiscibility is the difference in the polarity of the molecules that comprise each liquid phase. Usually, one phase is relatively lipophilic or non-polar, while the other is relatively hydrophilic or polar.
If the phases (e.g., water and oil) are combined without any agitation, they usually (although not necessarily) form two separate layers with a single phase-separation boundary between the layers. Emulsification typically involves agitation to disperse one phase throughout the other. Such dispersion leads to an increase in the area of surface contact between the phases, which have drastically different polarities. It is believed that the increase in the area of surface contact increases surface tension or surface energy of the phases.
For this reason, most emulsions are thought to be thermodynamically unfavorable and therefore inherently unstable after formation. To minimize the area of surface contact and the surface energy when left alone over time, emulsions tend to separate back into two layers with a single phase-separation boundary.
Since the products formulated as emulsions are usually stored for substantial periods of time prior to use, this separation of phases is highly undesirable. To prevent phase separation, formulators usually add various emulsifiers. The emulsifiers are surface-active compounds or surfactants that are believed to reduce the surface tension of the phases, thus reducing the thermodynamic drive for phase separation and improving the stability of emulsions.
The emulsifiers are usually compounds having both hydrophilic and lipophilic groups in the molecule. Somewhat oversimplifying, the hydrophilic groups contact the hydrophilic phase of the emulsion, and the lipophilic groups contact the lipophilic phase, in effect tying the phases together and acting to prevent phase separation. The selection and utilization of emulsifiers is one of the challenges in formulating emulsions.
Quaternary compounds
Personal care and cosmetic products often contain various surfactants, including cationic surfactants having a quaternary nitrogen atom (or quats). Quats are commonly used in hair care products. It is known that hair may be negatively affected by atmospheric agents, chemical treatments, and the like, resulting in damaged hair, which lacks softness and which may be difficult to disentangle or style. Quats facilitate disentangling and combing of hair, and provide softness to the hair, and thus they are used as active ingredients of hair care products, such as hair conditioners and conditioning shampoos.
Various types of quats are known in the art. Ammonium quats that contain quaternized ammonium nitrogen are described, for example, in U.S. Pat. Nos. 4,954,335 and 4,891,214. Examples of such quats include diphenyl dimethyl ammonium quat sold under the trade name INCROQUAT DBM-90 from Croda Incorporated, 7 Century Drive, Parsippany, N.J. 07054. Other examples of ammonium quats used in hair care products are cetyltrimethylammonium bromide and behenyltrimethylammonium bromide.
Other types of quats are also known in the art. Immidazoline-based quats containing the immidazoline ring are disclosed, for example, in U.S. Pat. Nos. 4,851,141, 4,452,732, 4,247,538, 4,206,195, 4,187,289, 4,149,551, and 4,102,795.
U.S. Pat. No. 4,102,795 discloses compositions for softening fabrics or hair that include immidazoline-based quarternary compounds of the formula where R′ and R′″ are alkyl groups having 11 to 22 carbon atoms, or β-hydroxyalkyl groups having from 13 to 24 carbon atoms; R″ is an alkyl group having 1 to 3 carbon atoms, benzyl group, or the group —(C2H4O)nH, where n is 1 to 3; and Y− is halogen or monoalkyl sulfate. The '795 patent mentions the possibility of mixtures of these immidazoline-based compounds, but does not provide their content or other information about such mixtures.
U.S. Pat. No. 4,452,732 discloses a shampoo containing several components, including immidazoline-based quaternary compounds of the formula where at least one of, and preferably both of the groups Q′ and Q′″ is/are a hydrocarbon group(s) containing 16 to 22 carbon atoms, preferably, 16 to 18 carbon atoms; Q″ is C1-C4 alkyl or hydroxyalkyl group; and Y− is a compatible anion. Mixtures are mentioned, but again, the mention is without information about their content, and no specific mixtures are disclosed.
Primarily, the immidazoline-based quats and the ammonium quats are disclosed as pure compounds. While mixtures of quats or mixtures with different surfactants had been mentioned in general, the prior art provides no specific mixtures or other information about the mixtures. Also, the prior art does not provide information about the relationships between primary product performance, such as performance in hair conditioning products, and emulsification characteristics of quats and/or quat mixtures in emulsion-formulated products.